AgentPay Sentinel MCP

A watchdog MCP that validates every agent payment request against all 9 security checks before it executes — catching policy violations, replay attacks, amount tampering, revoked tokens, and budget overruns at call time.

What your agent can do

Run a pre-flight audit on any payment transaction before executing it — get PASS or BLOCKED with the specific check that failed
Validate token integrity: confirms the SHA-256 hash matches the expected merchant_id:amount binding, catching forged or tampered tokens
Enforce budget caps: rejects transactions where current_spend + amount > budget_cap and returns exact remaining budget
Check merchant allowlist membership and block purchases in restricted categories
Verify token expiry and rate limits before the payment fires
Simulate known attack vectors (token forgery, budget overflow, replay, merchant spoof, expiry bypass) and get the specific defense mechanism and detection method for each

Installation

Requires: Python 3.10+, mcp package.

pip install mcp

Claude Desktop — add to ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%\Claude\claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "agentpay-sentinel": {
      "command": "python",
      "args": ["/absolute/path/to/agentpay-sentinel-mcp/server.py"]
    }
  }
}

Cursor — add to .cursor/mcp.json in your project root:

{
  "mcpServers": {
    "agentpay-sentinel": {
      "command": "python",
      "args": ["/absolute/path/to/agentpay-sentinel-mcp/server.py"]
    }
  }
}

Tool Reference

Tool	Description	Key params
`sentinel_audit_transaction`	Run all 9 security checks; returns PASS/BLOCKED + SHA-256 audit hash	`token_hash`, `merchant_id`, `amount`, `current_spend`, `budget_cap` (required) · `nonce`, `token_id`, `approved_amount` (enable checks 7–9)
`sentinel_revoke_token`	Permanently revoke a token — all future audits with this `token_id` will BLOCK	`token_id`, `reason`
`sentinel_clear_nonce`	Remove a nonce from the replay store (for legitimate refunds/retries only)	`nonce`
`sentinel_verify_chain`	Verify a sequence of audit hashes forms an unbroken chain	`audit_hashes`, `expected_chain_root`
`sentinel_threat_model`	Simulate any named attack vector; returns severity, defence, and detection	`attack_vector`, `context`

All 9 checks run by `sentinel_audit_transaction`

#	Check	What it catches	Param
1	Token integrity	SHA-256 hash mismatch — forged tokens	`token_hash`
2	Budget enforcement	Spend exceeding cap	`current_spend`, `budget_cap`
3	Merchant allowlist	Payment to unlisted merchant	`allowlist`
4	Category restriction	Purchase in blocked category	`blocked_categories`
5	Expiry check	Expired token	`expires_at`
6	Rate limit	Too many calls per minute	`calls_this_minute`
7	Amount mismatch	Agent changed amount after human approved	`approved_amount`
8	Replay attack	Same nonce used twice (file-backed store)	`nonce`
9	Revocation evasion	Agent using a revoked token	`token_id`

Checks 7–9 activate when the corresponding param is passed. State persists to ~/.sentinel/.

Attack vectors in `sentinel_threat_model`

token_forgery · budget_overflow · replay_attack · amount_mismatch · revocation_evasion · merchant_spoof · expiry_bypass

Security

sentinel_audit_transaction returns an advisory verdict — it does not intercept network traffic. Your agent is responsible for calling it before executing a payment and halting on BLOCKED. The audit hash returned is a SHA-256 digest of all check results, giving you a tamper-evident record of each pre-flight decision.

Pricing

Plan	Price	Included
Free	$0	50 audits/month
Pro	$19/month	Unlimited audits + threat model simulations

Upgrade to Pro

License

MIT — AgentPay Labs. Source: github.com/Rumblingb/agentpay-sentinel-mcp